This work was supported by grants from the Kempe Minne Foundation (BVB), the Swedish National Research Council (SW), Bioteknik Medel Umeå University (SW), the DFG (GR1211/2-4, En111/11-3) (C.G., H.S.), the Karl Winnacker foundation (H.S.), and the European LSF for Biomolecular NMR (ER-BCT950034). We thank Janny Hof for critical reading of the manuscript.